1. Field of the Invention
This invention relates generally to an electrical discharge machining apparatus, and more particularly to an electrical discharge machining apparatus adapted to obtain a high machining accuracy by providing a shielding cover for forming a substantially enclosed, predetermined space around main structural members to prevent the main structural members from being deformed by the thermal effects of discharge energy and the changes in the room temperature.
2. Description of the Prior Art
The electrical discharge machining apparatus, or simply the discharge machining apparatus, is usually classified into a wire type discharge machining apparatus using a wire electrode, as shown in FIG. 4, and a discharge machining using an electrode, made of graphite, for example, formed into a shape corresponding to a shape to be machined (hereinafter referred to as a profile discharge machining apparatus--not shown). As is widely known, both the wire type discharge machining apparatus and the profile discharge machining apparatus have essentially the same machining principle and machine construction consisting of a bed, a column and other main structural members.
FIG. 4 shows a basic construction of the wire type discharge machining apparatus of the conventional type. In the figure, reference numeral 1 refers to a bed; 2 to a column; 3 to an upper arm; 4 to a lower arm; 5 to a wire electrode; 6 to an electrode pay-off reel; 7 to a take-up reel; 8 to an upper guide; 9 to a lower guide; 10 to a UV table driven by a control unit (not shown) for controlling the position of the upper guide 8; 11 to an XY table driven by a control unit (not shown) in two orthogonally intersecting directions; 12 to a workpiece; and 13 to a workpiece support, respectively.
The wire type discharge machining apparatus shown in FIG. 4 performs discharge machining of the workpiece 12 by supplying discharge energy from a power supply (not shown) to cause an electric discharge between the wire electrode 5, which travels as it is wound up by the take-up reel 7, and the workpiece 12. Needless to say, a working fluid (distilled water, for example) is fed to the machining part of the apparatus from a working fluid supply unit (not shown).
In the discharge machining described above, a desired shape is machined by performing the positional control of the workpiece 12 on the XY table 11 and the positional control of the upper guide 8 on the UV table 10 by means of an NC and other appropriate control means.
It is no exaggeration to say that the machining accuracy of the wire type discharge machining apparatus shown in FIG. 4 depends solely on how accurately the relative positions of the upper guide 8, the lower guide 9 and the workpiece 12 are controlled. However, the bed 1, the column 2, the upper arm 3, the lower arm 4 and other main structural members of the conventional electrical discharge machining apparatuses, such as the wire type discharge machining apparatus shown in FIG. 4, have heretofore been fabricated by casting, sheet metal welding and other metal-working processes. As a consequence, these main structural members are subject to the deformation (expansion and shrinkage) caused by temperature changes. For this reason, a high machining accuracy cannot be maintained because deformation of the upper arm 3, the lower arm 4, the column 2 or any other main structural member causes a relative positional displacement between the upper guide 8 and the lower guide 9 no matter how accurately the UV table 10 and the XY table 11 are controlled. The following factors are as possible sources for the temperature changes that cause deformation of the main structural members.
(i) Eddy-current losses caused by discharge current in the main structural members that are magnetic materials. PA1 (ii) Changes in the room temperature. PA1 (iii) Increased temperature of working fluid due to discharge energy. PA1 (iv) Heat generated by electrical components used.
In general, electrical discharge machining apparatuses are operated continuously for long hours (for a few days, for example). The present Applicant measured temperatures of the room temperature, and the upper arm, lower arm, column, and working fluid of a wire discharge machining apparatus of the conventional type that was operated for a few days (measurement results are shown in FIG. 5). In FIG. 5, arrow a (.DELTA.---.DELTA.) indicates the temperature change in the upper arm, b (.cndot.---.cndot.) that in the lower arm, arrow c (.quadrature.---.quadrature.) that in the column, d (X---X) that in the working fluid, and e (o---o) the change in the room temperature, respectively. As to machining errors, measurements ranging from +15 .mu.m to -20 .mu.m in the X direction, and from +5 .mu.m to -10 .mu.m in the Y direction were obtained. Those machining errors were found occurring irregularly with the lapse of time.
According to the measurements, the temperatures of various parts of the apparatus change rather irregularly, with the consequence that the deformation of the upper arm, lower arm, column, etc. takes place irregularly. This leads to the failure to ensure consistent machining accuracy, as noted earlier.
The discharge machining apparatus of the conventional type, main structural members of which are usually made of cast metal parts or welded metal sheets, tends to cause distortion after the lapse of time, posing an unwanted problem of secular changes.
Furthermore, the discharge machining apparatus of the conventional type involves not only the aforementioned deformation of the main structural members but also lowered machining performance due to the deteriorated discharge current waveform resulting from the main structural members made of magnetic materials.